The last important contribution to the secular variation is the slow deformation of the Nylon Vessel over time. The shape of the vessel is precisely monitored through the background contamination present on its surface [1]}. Considering a standard polar coordinate system, the distance of the vessel surface from the center \(d(\theta ,\phi )\)  deviates slightly from its nominal value (\(r_0=4.25\)  m). It is observed that in the period of this analysis, the vessel displays a slow, monotonic deformation along \(\theta \)  while preserving azimuthal (\(\phi \) ) symmetry. Such a slow deformation can subtly affect the contribution from external \(\gamma \) -ray background
in the RoI.
The detrending procedure used to filter out these secular contributions is discussed in the next Section.
